For a static website, you should use a static site builder such as Jekyll or Eleventy. These are frameworks that you use to centralise your site, and they are an excellent way to keep your site consistent. I prefer Jekyll, mostly because I found it was very easy to set up on Linux. This site was developed using Jekyll.
Set Jekyll up, preferably on Linux or MacOS. If you have Windows, I recommend installing Virtualbox and running Linux as a virtual machine. The reason is that Linux and MacOS are a more natural and intuitive programming environment and work really well with the tools. The documentation on installing Jekyll assumes you are working in a Linux environment. This was easy for me because I have always preferred not to use Windows.
If you are working on Jekyll and some of your raw html code is being displayed on the browser window, it means there is an error in your HTML code - the parser checks for correct code and isn't as forgiving as a web browser. If you see code in the browser, there is an error in your code.